Autoimmunity is a multigenic disease that generally involves the presence of the pro- autoimmune allele at numerous genetic loci. There is a strong association between a single nucleotide polymorphism (SNP) in the PEST-domain containing intracellular protein tyrosine phosphatase, PTPN22 R620W, and numerous autoimmune diseases, including systemic lupus erythematosus, systemic sclerosis, rheumatoid arthritis, Graves diseases and Type 1 Diabetes. It is not understood how this allele contributes to these diseases. Type 1 diabetes (T1D) is increased in incidence by 2-4 fold when this allele is present. We propose to use the non-obese diabetic (NOD) mouse model of spontaneous type 1 diabetes (T1D) to determine how the mouse orthologue of R620W, R619W, contributes to disease. This model is ideal as many of the genes identified by genome- wide association studies in diabetic patients are the same or on the same pathway as those that contribute to T1D in NOD mice, including MHC, insulin, CTLA-4, and the IL- 2R pathway. The presence of the pro-autoimmune alleles may be critical to assess the impact of the R619W allele on T1D. We have successfully used the Crispr-Cas9 technology to mutate NOD fertilized embryos and produce 2 independent lines of NOD mice that express the R619W variant. We have also obtained several lines that incorporated insertions or deletions (indels) that caused frameshift mutations in this gene. We have ascertained all mutations are germline transmitted, and now propose to determine how they affect PTPN22 expression, immune cell development and homeostasis, and most importantly, disease progression. To this end we will pursue 2 Specific Aims. Aim1 will assess the level of expression of the mutant PTPN22 in various immune cell types from the independently derived NOD murine lines. The primary goal of Aim 2 is to determine whether the R619W and the indel mutants alter the incidence and progression of T1D. We will also assess its effect on immune cell development, function and homeostasis in NOD mice.